1. Field of the Invention
The present invention relates to instruments used in performing orthopaedic surgery, and more particularly, to an instrument used to aid in properly positioning a prosthetic ball on a bone in a ball joint, such as the replacement of a femoral head during hip replacement surgery.
2. Description of the Related Art
When performing joint reconstruction surgical procedures, such as hip replacement surgery, it is important that the pre-surgical geometry of the bone structure be replicated in the post-surgical structure, including prosthetic implants. Duplicating the geometric relationships maintains the natural joint biomechanics, ensuring proper joint and soft tissue balancing forces. If, during hip replacement surgery, for example, the center of the femoral prosthetic implant is moved slightly to a position different from the original position of the natural femoral head, the result can be higher joint forces, weak abductor muscles and overall joint instability. Even relatively slight mis-positioning of the prosthetic joint component can lead to a negative surgical result.
While planning the procedure, x-rays can be used for pre-surgical templating, to assist in determining the apparent appropriate implant size, head offset and head neck length necessary to reestablish the joint biomechanics. However, while pre-surgical, two dimensional templating is useful in selecting the proper prosthetic implant, and for establishing the proper fit and positioning of the implant during surgery, even with careful and deliberate pre-surgical planning and calculation, it is common to also perform intra-operative verification of the natural joint geometry and of the geometry resulting after implant, to ensure proper function of the joint post-surgically.
It is known during hip surgery to determine the natural offset and neck length of the femoral head by measurement. Offset can be measured from the tip of the greater trochanter to the center of the femoral head. Neck length can be measured from the tip of the lesser trochanter to the center of the femoral head. When selecting the prosthetic components, an attempt is made to duplicate as closely as possible the same measurements taken with respect to the prosthetic devices, after the provisional implants have been positioned. While placing the prosthetic components, the measurements are repeated, to verify concurrence between the pre-surgical and post-surgical geometry of the joint, by reestablishing the measured values in the joint after implant.
Even with advance planning, selection of the appropriate modular head and neck components of the prosthetic implant can require the trial of several components, with the proper components being identified only after repeated measurements of the several components tried. Various measuring calipers, frames or positioning jigs have been suggested as ways to increase the accuracy in duplicating the pre-surgical geometry of the joint after implanting prosthetic components.
What is needed is an orthopaedic instrument, and a surgical procedure for use of the instrument, which can be used to precisely and accurately duplicate, in the post-surgical geometry, the geometry that existed in the joint, pre-surgically.